Classical capacitance proximity gauging techniques typically measure the capacitance to a grounded conductor. It may frequently occur, however, that there is a need to capacitively gauge the proximity of an element which presents an impedance to circuit ground, such as might be the case in gauging proximity to resistive semiconductor wafers or the proximity to a moving foil in a continuous process. In such instances, it becomes difficult or impossible to define the potential of the element because of inherent element impedance or difficulty in securely grounding the element being gauged.
Capacitive thickness gauging is a specific example where the need arises to gauge the proximity of elements whose potential is undefined. These typically operate upon the basis of two opposed proximity gauges whose output indications of distance are combined to provide an indication of the total distance which reflects the thickness of the element. Where the element is ungrounded or of significant impedance, however, its electrical potential will be a function of the currents applied to the element through the two capacitors from the respective proximity probes. This potential will vary with the placement of the element between the two probes and with changes in element impedance to ground and may thus create a substantial uncertainty both of absolute and relative thickness being gauged.